In certain cases, aspects of the environment can disturb the correct operation of RCDs:

“nuisance” tripping: Break in power supply without the situation being really hazardous. This type of tripping is often repetitive, causing major inconvenience and detrimental to the quality of the user's electrical power supply.

non-tripping, in the event of a hazard. Less perceptible than nuisance tripping, these malfunctions must still be examined carefully since they undermine user safety. This is why international standards define 3 categories of RCDs according to their immunity to this type of disturbance (see below).

Main disturbance types

Permanent earth leakage currents

Every LV installation has a permanent leakage current to earth, which is either due to:

Unbalance of the intrinsic capacitance between live conductors and earth for three-phase circuits or

Capacitance between live conductors and earth for single-phase circuits

The larger the installation the greater its capacitance with consequently increased leakage current.

The capacitive current to earth is sometimes increased significantly by filtering capacitors associated with electronic equipment (automation, IT and computer-based systems, etc.).

In the absence of more precise data, permanent leakage current in a given installation can be estimated from the following values, measured at 230 V 50 Hz:

Single-phase or three-phase line: 1.5 mA /100m

Heating floor: 1mA / kW

Fax terminal, printer: 1 mA

Microcomputer, workstation: 2 mA

Copy machine: 1.5 mA

Since RCDs complying with IEC and many national standards may operate under, the limitation of permanent leakage current to 0.25 IΔn, by sub-division of circuits will, in practice, eliminate any unwanted tripping.

For very particular cases, such as the extension, or partial renovation of extended IT-earthed installations, the manufacturers must be consulted.

High frequency components

High frequency components (Harmonics, transients, etc.) are generated by computer equipment power supplies, converters, motors with speed regulators, fluorescent lighting systems and in the vicinity of high power switching devices and reactive energy compensation banks.

Part of these high frequency currents may flow to earth through parasitic capacitances. Although not hazardous for the user, these currents can still cause the tripping of differential devices.

Energization

The initial energization of the capacitances mentioned above gives rise to high frequency transient currents of very short duration, similar to that shown in Figure F67.

The sudden occurrence of a first-fault on an IT-earthed system also causes transient earth-leakage currents at high frequency, due to the sudden rise of the two healthy phases to phase/phase voltage above earth.

Fig. F67: Standardized 0.5 µs/100 kHz current transient wave

Common mode overvoltages

Electrical networks are subjected to overvoltages due to lightning strikes or to abrupt changes of system operating conditions (faults, fuse operation, switching, etc.).
These sudden changes often cause large transient voltages and currents in inductive and capacitive circuits. Records have established that, on LV systems, overvoltages remain generally below 6 kV, and that they can be adequately represented by the conventional 1.2/50 μs impulse wave (see Fig. F68).

Fig. F68: Standardized 1.2/50 µs voltage transient wave

These overvoltages give rise to transient currents represented by a current impulse wave of the conventional 8/20 μs form, having a peak value of several tens of amperes (see Fig. F69).The transient currents flow to earth via the capacitances of the installation.

Fig. F69: Standardized current-impulse wave 8/20 µs

Non-sinusoidal fault currents : RCDs Type AC, A, B

Standard IEC 60755 (General requirements for residual current operated protective devices) defines three types of RCD depending on the characteristics of the fault current:

Type AC

RCD for which tripping is ensured for residual sinusoidal alternating currents.

RCD for which tripping is ensured: - as for type A, - for pure direct residual currents which may result from three-phase rectifying circuits.

Cold

In the cases of temperatures under - 5 °C, very high sensitivity electromechanical relays in the RCD may be “welded” by the condensation – freezing action.
Type “Si” devices can operate under temperatures down to - 25 °C.

Atmospheres with high concentrations of chemicals or dust

The special alloys used to make the RCDs can be notably damaged by corrosion. Dust can also block the movement of mechanical parts.

See the measures to be taken according to the levels of severity defined by standards in Fig. F70.

Regulations define the choice of earth leakage protection and its implementation.
The main reference texts are as follows:

Standard IEC 60364-3:

- This gives a classification (AFx) for external influences in the presence of corrosive or polluting substances.
- It defines the choice of materials to be used according to extreme influences.

Immunity level for Schneider Electric residual current devices

The Schneider Electric range comprises various types of RCDs allowing earth leakage protection to be adapted to each application. The table below indicates the choices to be made according to the type of probable disturbances at the point of installation.

Device type

Nuisance trippings

Non-trippings

High frequency leakage current

Fault current

Low temperatures (down to - 25 °C)

Corrosion Dust

Rectified alternating

Pure direct

AC

A

SI

B

Fig. F71: Immunity level of Schneider Electric RCDs

Immunity to nuisance tripping

Type SI RCDs have been designed to avoid nuisance tripping or non-tripping in case of polluted network, lightning effect, high frequency currents, RF waves, etc.

Figure F72 below indicates the levels of tests undergone by this type of RCDs.

Recommendations concerning the installation of RCDs with separate toroidal current transformers

The detector of residual current is a closed magnetic circuit (usually circular) of very high magnetic permeability, on which is wound a coil of wire, the ensemble constituting a toroidal (or ring-type) current transformer.Because of its high permeability, any small deviation from perfect symmetry of the conductors encompassed by the core, and the proximity of ferrous material (steel enclosure, chassis members, etc.) can affect the balance of magnetic forces sufficiently, at times of large load currents (motor-starting current, transformer energizing current surge, etc.) to cause unwanted tripping of the RCD.Unless particular measures are taken, the ratio of operating current IΔn to maximum phase current Iph (max.) is generally less than 1/1,000.This limit can be increased substantially (i.e. the response can be desensitized) by adopting the measures shown in Figure F73, and summarized in Figure F74.

Fig. F73: Three measures to reduce the ratio IΔn/Iph (max.)

Measures

Diameter (mm)

Sensitivity diminution factor

Careful centralizing of cables through the ring core

3

Oversizing of the ring core

ø 50 → ø 100

2

ø 80 → ø 200

2

ø 120 → ø 300

6

Use of a steel or soft-iron shielding sleeve

ø 50

4

Of wall thickness 0.5 mm

ø 80

3

Of length 2 x inside diameter of ring core

ø 120

3

Completely surrounding the conductors and overlapping the circular core equally at both ends

ø 200

2

These measures can be combined. By carefully centralizing the cables in a ring core of 200 mm diameter, where a 50 mm core would be large enough, and using a sleeve, the ratio 1/1,000 could become 1/30,000.

Rated current

The rated current of a RCCB is chosen according to the maximum sustained load current it will carry.

If the RCCB is connected in series with, and downstream of a circuit-breaker, the rated current of both items will be the same, i.e. In ≥ In1 (see Fig. F75a)

If the RCCB is located upstream of a group of circuits, protected by circuit-breakers, as shown in Figure F75b, then the RCCB rated current will be given by: In ≥ ku x ks (In1 + In2 + In3 + In4)

Fig. F75: Residual current circuit-breakers (RCCBs)

Electrodynamic withstand requirements

Protection against short-circuits must be provided by an upstream SCPD (Short-Circuit Protective Device) but it is considered that where the RCCB is located in the same distribution box (complying with the appropriate standards) as the downstream circuit-breakers (or fuses), the short-circuit protection afforded by these (outgoing-circuit) SCPDs is an adequate alternative. Coordination between the RCCB and the SCPDs is necessary, and manufacturers generally provide tables associating RCCBs and circuit-breakers or fuses.